A complex sentence is formed by joining an independent clause with a subordinate clause. A subordinate clause always begins with a subordinating conjunction.
Here are some of the subordinating conjunctions: if, since, although, when, while
Anytime you begin a sentence with one of the subordinating conjunctions, you are writing a complex sentence.
If it stops raining, we can go to the zoo.
Since my bike has a flat tire, I have to walk to school.
Although the sun is shining, we are expecting rain this afternoon.
When I finish my homework, we can play some pickleball.
While you are studying, I will read a book.
A subordinate clause begins with a subordinating conjunction such as if, since, although, when, while.
A complex sentence is formed when you join an independent clause and a subordinate clause.
It’s your turn. Find the subordinating conjunction in this sentence. That word will be the beginning of the subordinate clause. If you help me, we can get the lawn mowed for Dad. Did you find the subordinating conjunction? Good for you!